In continuous ink jet printing, ink is supplied under pressure to a manifold region that distributes the ink to a plurality of orifices, typically arranged in a linear array(s). The ink discharges from the orifices in filaments which break into droplet streams. The approach for printing with these droplet streams is to selectively charge and deflect certain drops from their normal trajectories. Graphic reproduction is accomplished by selectively charging and deflecting drops from the drop streams and depositing at least some of the drops on a print receiving medium while other of the drops strike a drop catcher device. The continuous stream ink jet printing process is described, for example, in U.S. Pat. Nos. 4,255,754; 4,698,123 and 4,751,517, the disclosures of each of which are totally incorporated herein by reference.
In the ink jet printing art, water soluble dyes are desirable for runnability, but waterfast inks are highly desirable for producing permanent images. It is also known in the industry that a waterfast security ink is highly desired for printing important documents such as lottery tickets, checks, etc., to prevent alteration of information on such documents. Bleach is a powerful tool used to decolorize most inks since it attacks the dye molecule and oxidizes it into small colorless fragments.
Compounds such as N,N-diphenyl guanidine, which is believed to be the active component in Chlorostain BR (By Mobay Corp., Pittsburgh, Pa.), and Chlorostain OR which is also commercially available from Mobay and based on benzothiazole chemistry, are used as additives to formulate security inks.
One drawback for diphenyl guanidine and benzothiazole additives is that they are not water soluble and require the use of either potentially dangerous solvents such as methyl cellosolve and methyl carbitol, or the use of acids to dissolve them. It is well known that low pH fluids can attack Ni orifice plates and cause corrosion, which is extremely undesirable in the ink jet printing industry.
U.S. Pat. No. 5,304,587 disclosed the use of a mixture of these compounds to make secure inks. When Chlorostain OR (Benzothiazole) was used alone in 27% as demonstrated in Example 5, the ink failed the bleach security test. When both Chlorostain OR and BR were used in 27% as demonstrated in Example 4, (otherwise the rest of the ingredients remain the same in both Examples 4 and 5), the ink of Example 4 passed the bleach security test. The '587 patent, then, teaches that 27% of both Chlorostain OR and BR are needed to formulate security inks.
However, the percent solids in ink jet inks is usually kept below 10%, and preferably below 5%, to insure good runnability of the inks in the printer. Percent solids above these values raise viscosity of the ink which, in turn, adversely affects stimulation and jet straightness. It is desirable to keep viscosity of the ink below 2 Centipoise, and preferably at or below 1.5 Centipoise at ambient temperature. In addition, lower solid contents in the ink would improve redisolvability of the dried ink on the orifice plate.
It is seen then that there is a great need for inks which do not bleach out completely, but leave a visible stain behind to prevent attempts at altering of information on a document.